The first step in DNA base excision repair is catalyzed by a class of enzymes, DNA glycosylases, which promote the scission of the N-C1 1 - glycosyl bond linking a damaged or mismatched base to the DNA sugar phosphate backbone. Following exposure of DNA to ultraviolet light, T4 endonuclease V initiates the repair of cyclobutane pyrimidine dimers by way of specific glycosylase and AP lyase activities. Although the catalytic mechanism is relatively well-understood, the events occurring prior to the cleavage of the glycosyl bond are less well defined. Using putative transition state and substrate analogs, we propose to examine the structural features of both the DNA and the reaction intermediates which determine the specificity of T4 endonuclease V binding. In addition, the key features of the protein architecture that are required for differential binding, base flipping, and catalysis will be elucidated by use o site-directed mutants. These studies will provide pertinent biochemical and structural information regarding the generalized mechanisms of DNA glycosylases, and the use o transition state analogs for mechanistic and inhibition studies of DNA repair enzymes.